Resin-cemented optical element, mold therefor, fabrication process thereof, and optical article

ABSTRACT

The present invention provides a resin-cemented optical element comprising a base member  10  and a resin layer  11  formed on the surface of the base member; the resin layer  11  being in a thickness of 300 μm or smaller at least at some part of a peripheral portion (i.e., a region within 1 mm from the peripheral edge face  17  of the resin layer  11 , or a region outside an effective-diameter region), and being in a thickness  12  of 850 μm or larger at a position which is thickest in the resin layer; a mold therefore; a manufacturing method thereof; and an optical article having this optical element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of application Ser. No.09/995,832, filed Nov. 29, 2001 now abandoned, which claims the benefitsof priority Japanese Patent Application No. 2000-365992 filed Nov. 30,2000, and priority Japanese Patent Application No. 2001-231933 filedJul. 31, 2001. The contents of U.S. application Ser. No. 09/995,832,Japanese Application No. 2000-365992, and Japanese Application No.2001-231933 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a resin-cemented optical element, a mold usedfor producing the element, an optical article (or device) having theelement, and fabrication process of the element

2. Description of the Related Art

At present, optical elements are used in various fields. Depending onthe purpose for which they are used, it is difficult to materializerequired optical characteristics and so forth in some cases in respectof conventional spherical lenses. Accordingly, aspheric lenses areattracting notice. “Aspheric lens” is a generic term for lenses thecurvature of which is kept continuously different over the regionextending from the lens center toward the periphery. The use of asphericlenses at some part of optical systems enables considerable reduction ofthe number of lenses necessary for the correction of aberrations,compared with a case where the optical system is comprised of onlyspheric lenses. This enables downsizing and weight reduction of theoptical system. Also, the use of aspheric lenses enables high-gradecorrection of aberrations which is difficult for spherical lenses, andhence can bring about an improvement in image quality.

Aspheric lenses having such superior characteristics have notnecessarily come into wide use. The greatest reason therefor can be saidto be a difficulty in working. Conventional aspheric lenses have only beable to be produced by precisely polishing base members made of glass,and have involved the problem of a high processing cost.

In recent years, however, resin-cemented optical elements thatmaterialize aspherical shapes by the aid of resin layers which can bemade into any desired shapes with much greater ease than the precisepolishing of glass have been put into practical use, so that asphericlenses have rapidly come into wide use.

The resin-cemented optical element is an element in which a resin layerhas been cemented to the surface of a base member made of glass or thelike. This resin-cemented optical element is produced by a process suchas a composite-type aspherical-surface molding process, in which, usinga mold (such as a metal mold), a resin composition (inclusive of a resinprecursor composition) is poured into a space between a base member andthe mold, followed by curing to form on the base member surface a resinlayer having any desired shape. In the present specification, a lensproduced by this composite-type aspherical-surface molding process maybe called a PAG (plastic adhesion glass) lens.

In the case when the resin-cemented optical element is thus produced bythe composite-type aspherical-surface molding process, the base membermay break when the resin cured on the base member is released from themold. This phenomenon is remarkable especially when the resin layer hasa large thickness. Accordingly, it has been impossible in practice toproduce any PAG lens having a thick resin layer of 850 μm or larger inmaximum layer thickness.

This phenomenon is considered to be caused by the adhesion of the resinlayer to the mold. Usually, the resin is released from the mold by meansof an ejector (ejection member) in such a way that a force acting in thedirection where the former is released from the latter is applied to thebase member at its part standing uncovered to the periphery of theelement. Here, in the event that the resin layer remains without beingreleased, in the state it has been kept adhered to the mold until theamount of deformation of the base member exceeds a tolerance limit, thebase member breaks because of the distortion due to a deformation havingexceeded the tolerance limit.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide aresin-cemented optical element having a thick resin layer, withoutcausing any break of the base member, and to provide a mold used forproducing the element and an optical article having the element.

To achieve the above object, the present invention provides aresin-cemented optical element comprising a base member and a resinlayer formed on the surface of the base member, wherein the resin layeris in a thickness of 300 μm or smaller at least at a part of aperipheral portion (i.e., a region within 1 mm from the peripheral edgeface of the resin layer, or a region outside an effective-diameterregion), and is in a thickness of 850 μm or larger at a position whichis thickest in the resin layer.

The present invention also provides a mold for forming a resin layer ofa resin-cemented optical element having a base member and a resin layerformed on the surface of the base member, wherein the mold has, on theouter periphery on the outside of a molding surface, a concavely curvedsurface which has a curvature larger than the molding surface. It stillalso provides an optical article having the resin-cemented opticalelement of the present invention and a fabrication process of theelement.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view showing an example of the constructionof the optical element according to the present invention;

FIG. 2 is a cross-sectional view of an optical element having a stair;

FIG. 3 is an illustration showing an angle at which a normal of the basemember surface falls with a resin layer tangent plane;

FIG. 4 is a cross-sectional view showing an example of the constructionof the optical element according to the present invention;

FIGS. 5A and 5B illustrate the steps of producing an optical element inExample 1;

FIG. 6 is a graph showing resin thickness in the optical elementproduced in Example 1;

FIG. 7 is a cross-sectional view showing a mold used in Example 2;

FIG. 8 is a cross-sectional view showing a peripheral portion of moldingsurface of the mold used in Example 2; and

FIGS. 9A to 9C illustrate the steps of producing a mold used in Example2.

DETAILED DESCRIPTION OF THE INVENTION

In the resin-cemented optical element of the present invention, as shownin FIG. 1, a resin layer 11 has a thickness of 300 μm or smaller(preferably 100 μm or smaller) at least at some part of a peripheralportion (i.e., a region within 1 mm from the peripheral edge face 17 ofthe resin layer 11, or a region outside the effective-diameter region),and has a thickness of 850 μm or larger (preferably 1 mm or larger) asthe maximum value of the thickness of the resin layer 11. Also, in orderto attain necessary strength, optical characteristics and so forth, theresin layer 11 may preferably be formed usually in a thickness of atleast 20 μm, without regard to the inside or outside of the peripheralportion. Incidentally, what is shown in FIG. 1 takes the case of anoptical element whose resin layer molding surface is convex, to which,however, the present invention is by no means limited.

The resin layer may have the thickness of 300 μm or smaller at its wholeperipheral portion, but may be enough as long as it has the thickness of300 μm or smaller at least at some part of the peripheral portion. Thisis because the resin in the vicinity where a force for peeling isapplied at the time of mold release may have layer thickness in thisvalue. In the present invention, the resin present within 1 mm inperiphery from the resin layer edge face closest to the part to which aforce for mold release is to be applied (i.e., the part against which anejector is to be pressed) may be in the thickness of 300 μm or smaller.

Here, the peripheral portion is meant to be a region within 1 mm fromthe peripheral edge face 17 of the resin layer 11, or a region outsidethe effective-diameter region. A region inside the effective-diameterregion is meant to be a region through which light rays used in opticaldesigning are transmitted, thus the region outside theeffective-diameter region is meant to be a region except for thisregion. In general, the resin thickness of an element is strictlydetermined in accordance with the required optical characteristics.However, as long as it is in the region outside the effective-diameterregion, it does not affect any optical characteristics of the element.Hence, the layer thickness can appropriately be selected.

In the optical element of the present invention, the resin layer maypreferably have layer thickness which becomes gradually smaller towardthe periphery, at least at some part of the peripheral portion. Makingthe resin layer have such a thickness that does not form any stair so asnot to have any abrupt change in thickness is preferred because not onlymolds can be produced with easy but also any defects can be preventedthat may occur because the resin can not turn around when a resincomposition is poured into the mold.

According to the present invention, even a resin-cemented opticalelement having characteristic features a to k as shown below, havingbeen considered impossible in elements having the resin layer of 850 μmor larger in maximum layer thickness, can be produced in a good yieldwithout causing any break at the time of mold release.

a. As shown in FIG. 1, the resin layer 11 has a maximum layer thickness12 which is at least four times a minimum layer thickness 13.

b. The resin layer 11 has a total mass of 700 mg or larger.

c. The resin layer 11 has an external diameter 14 of 34 mm or larger.

d. The base member 10 has a thickness of 10 mm or larger as maximumvalue.

e. The base member 10 has a thickness of 1 mm or smaller as minimumvalue.

f. The base member 10 has an external diameter 15 of 35 mm or larger.

g. As shown in FIG. 2, the base member 10 has a resin layer 11 moldingsurface which is a concave surface, and the base member 10 has along itsperiphery a stair 16 which protrudes in the peripheral direction (e.g.,an attachment part for fastening the base member to a lens barrel).Incidentally, hatching is omitted in FIG. 2 in order to make theillustration easy to view.

h. As shown in FIG. 3, an angle 23 at which a normal 21 of the interface20 between the base member 10 and the resin layer 11 falls with atangent plane 22 on the outside of the resin layer is 80° or smaller asminimum value.

i. As shown in FIG. 4, the base member 10 has a resin layer 11 moldingsurface which is a concave surface, and the resin layer 11 has anexternal diameter 14 which is at least 1.2 times a curvature radius 31of the concave surface.

j. As shown in FIG. 4, the base member 10 has a resin layer 11 moldingsurface which is a concave surface, and the resin layer 11 moldingsurface has a curvature radius 31 of 24 mm or smaller.

k. The base member has a resin layer molding surface which is a convexsurface, and the resin layer has an external diameter which is at least1.2 times a curvature radius of the convex surface.

According to the present invention, a resin-cemented optical elementhaving a resin layer with a large maximum layer thickness can beobtained in a good yield.

There are no particular limitations on the base member used in theoptical element of the present invention. Sol-gel glass, inorganic glassand organic glass may be used. Usually used are transparent materialshaving a refractive index of nd=1.4 to 2.0 and νd=20 to 100 inapproximation. However, an opaque material or a semitransparent materialmay be used as the base member where the resin is not cured by exposureor, even when cured by exposure the resin can be exposed to light on theside of the mold.

Components constituting the inorganic glass may include, e.g., SiO₂,B₂O₃, P₂O₅, Na₂O, K₂O, CaO, BaO, MgO, ZnO, PbO, MnO, Al₂O₃ and Fe₂O₃.The organic glass may include poly(methyl methacrylate), polystyrene,poly(vinyl chloride), polyester, celluloid, and cellulose derivatives.

There are no particular limitations on the resin that constitutes theresin layer in the present invention, and any of photosensitive resins,thermosetting resins and thermoplastic resins may appropriately beselected as long as they can be molded by means of a mold. Thethermosetting resins, which are suited for the present invention, mayinclude, e.g., epoxy resins, urethane resins, thiourethane resins,unsaturated polyester resins, diallyl phthalate resins, and diethyleneglycol bisallyl carbonate known under a trade name CR-39. Also, thethermoplastic resins may include poly(methyl methacrylate), polystyreneand polycarbonate. Photosensitive acrylic resins and photosensitivemethacrylic resins are also preferable for the present invention.

A resin composition used in the optical element of the present inventionmay preferably have a viscosity before polymerization curing, of 50,000cP or lower at room temperature. If it has a viscosity higher than50,000 cP, a poor operability may result and besides some failure due toinclusion of bubbles may greatly occur.

The resin composition used in the present invention may appropriatelyoptionally contain, in addition to the resin (or a precursor thereof), apolymerizing agent (curing agent), a polymerization initiator, a releaseagent, an anti-scratching agent and so forth.

The polymerizing agent and the polymerization initiator mayappropriately be selected depending on the type of and curing conditionsfor the resin to be used, required film properties and so forth. As thereleasing agent, usable are, e.g., neutralizable or non-neutralizablephosphate alcohols. The anti-scratching agent has the effect ofsmoothing the surfaces of cured products to improve resistance toscratching, and keeping any faults from occurring. This anti-scratchingagent may include silicon oxides such as tetramethoxysilane,tetraethoxysilane, γ-methacryloxypropyltrimethoxysilane,γ-glycidyloxypropyltrimethoxysilane, and acrylate or methacrylate havingan Si—O bond at some part of the backbone chain.

The resin-cemented optical element of the present invention may include,e.g., lenses, prisms and diffraction gratings. The present invention canbring about superior effects especially when applied to aspheric lenses.The present invention may also be applied to aspheric mirrors.

In particular, the optical element of the present invention is suited tooptical articles (or devices) such as still cameras especially requiredto be made small-size and/or light-weight, such as analog still camerasand digital still cameras, video cameras, and interchangeable lenssystems for these cameras, as well as spectacles, telescopes,binoculars, microscopes and optical disk/magneto-optic disk readingpickup lens systems. Accordingly, the present invention also providesthese optical articles having the optical element of the presentinvention.

The resin layer of the resin-cemented optical element of the presentinvention can be formed by means of a molding tool the molding surfaceof which has an inverted shape of the resin shape described above.According to the molding of the resin layer of the resin-cementedoptical element of the present invention, the thickness of a resin wellformed on the outside of the resin layer outer edge at the time ofmolding can be made much smaller. Hence, in particular, it is preferableto use at a peripheral portion outside a molding surface (hereinafteroften simply “molding surface peripheral portion”) a mold having aconcavely curved surface which has a larger curvature than the moldingsurface. Accordingly, the present invention provides a mold for moldingthe resin layer of the resin-cemented optical element; the mold havingat its peripheral portion outside a molding surface a concavely curvedsurface which has a larger curvature than the molding surface. In such amold, the resin layer surface (its base member side being regarded asthe back) may be either of a concave surface and a convex surface. Itmay be used also for the molding of a lens having both the concavesurface and the convex surface. It is effective especially when used forthe molding of a concave lens (a lens the resin layer surface of whichhas a concave shape).

The concavely curved surface at the molding surface peripheral portion(hereinafter often simply “concavely curved surface”) may preferably beat least 0.1 mm outside the effective-diameter region, and morepreferably be at least 0.2 mm outside the effective-diameter region.Also, the edge of the curved surface (the position at which the curvedsurface begins as viewed on the inside of the effective-diameter region,i.e., the position at which the curvature changes from the curvature ofthe molding surface) may preferably be not distant by 0.5 mm or morefrom the effective-diameter region. The concavely curved surface maypreferably be so hollowed inward that a cross section embracing an axiscorresponding to the optical axis of the resin layer to be molded formsan inverted arc. Its curvature may appropriately be determined as longas it is larger than that of the molding surface, and may usually be 0.6to 1.5 mm in radius. The distance between the base member of the elementto be molded and the outer edge of the mold may preferably be so set asto be 1 mm or shorter.

The mold of the present invention may be produced by cutting orgrinding, depending on materials. For example, where the mold isproduced from a cuttable material such as electroless nickel plating(nickel formed by electroless plating), motion transfer type cutting maybe performed by the use of a cutting tool having a cutting surface witha small curvature. This enables formation of the concavely curvedsurface at the molding surface peripheral portion of the mold.

However, such a cuttable material, though moldable with ease, often hasa short lifetime because it tends to be scratched. Also, this methodrequires replacement of the cutting tool in the course of cutting,resulting in a high cost and besides tending to produce a difference inheight at the surface formed.

Accordingly, in order to achieve mass production of optical elements, itis preferable to produce the mold with use of a hard material such assingle-crystal silicon, SiC, CVD (chemical vapor deposition)-SiC, WC,SKD or hardened steel. These materials, however, can not be shaped bycutting, and must be shaped by grinding. Thus, in the method of motiontransfer type cutting, the cutting object and the cutting tool mayinterfere with each other, and hence any concavely curved surface havinga large curvature (i.e., having a small curvature radius) can not beformed.

Accordingly, in the case when the mold is produced by grinding, a formgrinding wheel having a grinding surface in an inverted shape of atleast part of the molding surface of the mold to be produced may be usedso that the shape of the grinding surface can be transferred. Thus, amold having the concavely curved surface having a large curvature can beobtained. According to the method in which the shape of the grindingsurface is transferred using the form grinding wheel in this way, even aconcavely curved surface having a small curvature radius (e.g., acurvature radius of 3 mm or smaller) can be formed at a low cost and asintended.

This form grinding wheel can be produced by, e.g., cutting a cuttablematerial such as brass to prepare a form grinding wheel base originally,and bonding abrasive grains to its grinding surface. Here, as theabrasive grains, it is preferable to use hard abrasive grains such asparticles of single-crystal or polycrystalline diamond or CBN (cubicboron nitride). Also, the abrasive grains may be bonded by plating witha nickel alloy or the like.

THE PREFERRED EMBODIMENTS

In the following Examples, the resin layer is irradiated by light(ultraviolet rays) on the side of the base member, and a mold made ofmetal is used as the mold. The present invention, however, is by nomeans limited to these. For example, a transparent material such asglass may also be used as the mold. In the case when such alight-transmitting material is used as the mold, the resin compositioncan be cured by irradiation on the mold side, and hence the base memberneed not be transparent.

Example 1

80 parts by weight of dimethacrylate represented by the followingstructural formula (1), having a weight-average molecular weight of 800,19.5 parts by weight of urethane-modified hexamethacrylate representedby the following structural formula (2) and 0.5 part by weight of anacetophenone type photoinitiator were mixed to prepare a photosensitiveresin composition,

wherein R is

As shown in FIG. 5, on the resin layer molding surface (concave surface)of a glass base member (BK7) of 40 mm in external diameter (diameter), 1mm in center thickness, 10 mm in maximum thickness and 18 mm in resinlayer molding surface curvature, having been subjected to silanecoupling treatment to improve adhesion to resin, the above resincomposition, 51, was dropped. The glass base member 10, with its upsidedown, was pressed against a convex surface of an aspheric metal mold 52to press and spread the resin composition 51 into the desired shape.Thereafter, on the base member side, the resin composition wasirradiated by ultraviolet rays 53 for 5 minutes by means of ahigh-pressure mercury lamp at an illumination of 10 mW/cm² to effectcuring to form a resin layer 11 having thickness distribution shown inFIG. 6.

Subsequently, the glass base member 10 was pushed with an ejector at theformer's peripheral portion 54 to release the resin layer 11 from themetal mold 52 to obtain a PAG lens. Here, the resin layer was formed inan external diameter of 38 mm, a maximum resin thickness of 850 μm, aresin thickness outside the effective-diameter region (within 1 mm fromthe peripheral edge of the resin layer), of 300 μm or smaller, and aresin quantity of 700 mg. The amount of deformation of glass at the timeof mold release was 20 μm.

The resin layer of the PAG lens obtained in the present Example has alarge aspherical shape in a maximum thickness of 850 μm and a minimumthickness of 100 μm. Even though the resin layer was molded in such alarge aspherical shape, the desired aspherical shape was exactlytransferred, and a PAG lens having a precise aspherical surface wasobtainable without any break of the base member at the time of moldrelease. Ten PAG lenses were produced in the same manner as the above.As the result, any break of the base member did not occur at all in allthe lenses.

Example 2

In the present Example, a PAG lens was molded using a metal mold havingthe concavely curved surface at the molding surface peripheral portion.A cross section of an aspherical surface metal mold 70, cut along aplane embracing an axis 71 (in the present Example, the axis ofrotation) corresponding to the optical axis of the lens to be molded, isshown in FIG. 7. An enlarged view of its peripheral portion 72 is shownin FIG. 8 in a state held at the time of molding.

The metal mold 70 used in the present Example has, as shown in FIGS. 7and 8, a concavely curved surface 73 at the molding surface peripheralportion. This concavely curved surface 73 is so formed that the crosssection embracing an axis 71 corresponding to the optical axis of theresin layer to be molded forms an inverted arc having a curvature radiusof 1 mm. The position 83 at which the curved surface begins as viewed onthe side of the effective-diameter region is kept at 0.3 mm outside theeffective-diameter region (diameter: 33.4 mm) of the lens to be molded.

According to the mold 70 of the present Example, the concavely curvedsurface 73 is provided at the molding surface peripheral portion. Hence,a resin well 81 can be made much thinner than a resin well 82 formed atthe time of molding when the metal mold 52 of Example 1 is used.

The metal mold 70 of the present Example was produced in the followingway. First, brass was cut to originally prepare a form grinding wheelbase 90 shown in FIG. 9A, and nickel alloy plating was applied to itsgrinding surface by the use of a plating solution mixed with abrasivegrains. Thus, as shown in FIG. 9B, a form grinding wheel 92 having aplating layer 91 having abrasive grains on its surface was obtained. Thegrinding surface of this form grinding wheel 92 has an inverted shape ofthe molding surface of the metal mold 70 to be ground. Morespecifically, a convexly curved surface 93 is provided at the innerperiphery of the grinding surface.

Subsequently, as shown in FIG. 9C, the grinding wheel 92 was rotated anda grinding fluid 96 was fed to the grinding surface, during which thesurface of the mold 70 on its molding surface side was pressed againstthe grinding surface of the grinding wheel 92 to transfer the shape ofthe grinding wheel 92 to the surface of the mold 70. Thus, the mold 70shown in FIGS. 7 and 8 was obtained, having the concavely curved surface73 at the peripheral portion on the outside of the molding surface.

PAG lenses were produced in the same manner as in Example 1 except thatthe aspherical mold 70 produced as described above was used in place ofthe aspherical mold 52. As the result, any break of the base member didabsolutely not occur in all the lenses.

Comparative Example 1

Ten PAG lenses were produced in the same manner as in Example 1 exceptthat the resin layer outside the effective-diameter region was in alayer thickness of 800 μm. The amount of deformation of glass at thetime of mold release was 80 μm on the average. Of the ten base members,five were seen to break.

While we have shown and described several embodiments in accordance withour invention, it should be understood that disclosed embodiments aresusceptible of changes and modifications without departing from thescope of the invention. Therefore, we do not intend to be bound by thedetails shown and described herein but intend to cover all such changesand modifications a fall within the ambit of the appended claims.

1. A resin-cemented optical element, comprising: a base member having anexternal diameter and a periphery, and a resin layer having an externaldiameter and a peripheral edge face, and said resin layer is formed on asurface of said base member, wherein: said base member and said resinlayer are in a concentric shape seen from an optical axis direction,said external diameter of said base member is larger than said externaldiameter of said resin layer, an exposed region of said base member thatis not covered with said resin layer is used to be pushed with anejector when said optical element is released from a mold, a peripheralportion of said resin layer that is a region within 1 mm from saidperipheral edge face has a thickness of 300 μm or smaller for at leastsome part of said peripheral portion, and a region of said resin layerthat is inside by 1 mm or more from said peripheral edge face has athickness of 850 μm or larger as maximum thickness of said resin layer.2. The optical element according to claim 1, wherein said base memberhas a maximum thickness of 10 mm or larger.
 3. The optical elementaccording to claim 1, wherein said base member has a minimum thicknessof 1 mm or smaller.
 4. The optical element according to claim 1, whereinsaid external diameter of said base member is 35 mm or larger.
 5. Theoptical element according to claim 1, wherein: said resin layer has amolding surface which is a concave surface, said exposed region of saidbase member that is not covered with said resin layer is a stair in saidperiphery that protrudes in the peripheral direction, and said stair isused as an attachment part for fastening said base member to a lensbarrel.
 6. A resin-cemented optical element, comprising: a base memberhaving an external diameter and a periphery, and a resin layer having anexternal diameter and a peripheral edge face, and said resin layer isformed on a surface of said base member, wherein: said base member andsaid resin layer are in a concentric shape seen from an optical axisdirection, said external diameter of said base member is larger thansaid external diameter of said resin layer, an exposed region of saidbase member that is not covered with said resin layer is used to bepushed with an ejector when said optical element is released from amold, an effective diameter region of said resin layer through whichlight rays used in said optical element are transmitted, an outsideregion of said resin layer which is outside of said effective diameterregion has a thickness of 300 μm or smaller for at least some part ofsaid outside region, and an inside region of said resin layer which isinside of said effective diameter region has a thickness of 850 μm orlarger as maximum thickness of said resin layer.
 7. The optical elementaccording to claim 6, wherein said base member has a maximum thicknessof 10 mm or larger.
 8. The optical element according to claim 6, whereinsaid base member has a minimum thickness of 1 mm or smaller.
 9. Theoptical element according to claim 6, wherein said external diameter ofsaid base member is 35 mm or larger.
 10. The optical element accordingto claim 6, wherein: said resin layer has a molding surface which is aconcave surface, the exposed region of the base member that is notcovered with said resin layer is a stair in said periphery, protrudingin the peripheral direction, and said stair is used as an attachmentpart for fastening said base member to a lens barrel.